Birds present an interesting dilemma for classical theories of sound localization. Because of their restricted range of hearing and closely-spaced ears, it is difficult to understand how birds can localize sound at all. Yet, there is clear evidence from playback studies in the field that birds can and do localize a variety of conspecific vocal signals as well as the vocal signals of other species. The present research plan is an ongoing effort to use behavioral techniques to measure Minimum Audible Angles (MAAs) for simple and complex (vocalizations) acoustic signals in an effort to define the acoustic cues by small birds in localizing sound. Preliminary evidence indicates that the budgerigar is remarkably sensitive to changes in spatial location of a pure-tone sound source--suggesting the interaural pathway may be involved. The canary is less sensitive than the budgerigar but shows a particular sensitivity to species-specific contact cells. Comparisons of several species selected for the range of interaural distances and the characteristics and function of their vocal signals should reveal the strategies used by small birds in localizing sound and may provide evidence that locatability is a factor in the design of some avian vocalizations. Calibration experiments will provide confirming evidence for the existence of binaural cues available for sound localization. The present experiments will provide proof of whether there is a new mechanism for localizing sound in small birds. Except for pilot data from this proposal (on budgerigars), there is no evidence that any organism uses an interaural pathway in sound localization. If the present proposal fails to show that budgerigars use the interaural pathway in sound localization, then the nervous system of this species must be extraordinarily adapted for coding binaural intensity in time differences.